1. Field of the Invention
The present invention is directed to the detection of radiation shielding material in a container, and, more particularly, to the detection of such material using an inverse ratio of gamma-ray to neutron radiation emission from a given container.
2. Background of the Art
In the post World War II era, global tensions festered and resolved within an environment dominated by the tremendous destructive capability of nuclear weapons that could kill millions of people and destroy large metropolitan areas in a single, momentary event. For several decades after 1945, a handful of nations, generally aligned within a bipolar world order, engaged in the proliferation of these weapons, continually increased the destructive capability of each warhead and refined delivery systems capable of launching a near instantaneous annihilative force with unprecedented stealth and accuracy.
Yet, despite the unprecedented military power ever possessed by a single nation state, the sheer magnitude of the mutual destruction that would result from the initiation of an attack by one state upon another, and the equally destructive response which would ensue, using these weapons prevented their very use. This concept of mutually assured destruction provided the delicate balance in the bipolar alignment of world order that, irrespective of the ever-increasing magnitude of destructive force on each side, such force would never be used.
Presently, however, that delicate balance has been lost. The collapse of bipolarity and the emergence of unipolarity have resulted in the emergence of rogue nations and terrorists as the primary threat to world order. Rogue nations generally comprise small quasi-failing states that have either developed nuclear weapons or have active nuclear weapons programs in progress. Terrorists are small non-national groups who use random and wanton destruction of human life and property to create fear and panic in the civilian population for political gains. In either event, rogue nations and terrorists are increasingly capable of buying nuclear weapons technology on the world's black-market. Once in possession of nuclear weapons, either merely needs a method of delivery to cause great harm.
As set forth in the '154 and '307 Applications, the Sep. 11, 2001 attack on the New York World Trade Center by a terrorist group known as al Qaeda was a wake-up call for the nations of the world. People suddenly realized that their countries and nations were vulnerable to attack by using the common transport mechanisms of global commerce as a weapons delivery system or as the weapons themselves. Accordingly, these transport mechanisms have been subject to increased monitoring and more stringent security measures to minimize the chances of weapons delivery. However, the magnitude of the increase in monitoring or of the stringency of security measures is inherently limited so that such monitoring and measures do not disadvantageously choke the stream of commerce thereby potentially resulting in economic disruption and instability. Such disruption and instability would nonetheless achieve the aims of the terrorist even if no further weapons were ever delivered.
One of the basic transport mechanisms of the modern global economy is containerized shipping. Through seaports and border crossings, shipping containers freely move in and out of the nations of the world with little or no inspection of their contents. For example, as of February 2004, the United States Government admits that ninety-five percent (95%) of the some 30,000 shipping containers that enter U.S. ports every day are not inspected in any way. The same is true of the millions of shipping containers annually brought into the United States across its borders with Canada and Mexico.
This lack of inspection extends in even greater percentages to the some 200 million shipping containers that move in and out of the ports, and over the roads and rails, of the nations of the world every year. It has now been clearly recognized that rogue nations or terrorists could potentially deliver a nuclear weapon by stealth in a shipping container destined for a particular population center.
One way to prevent the delivery of a nuclear weapon by stealth is to physically inspect each and every shipping container as it moves through all of the major transit points, that is at each highway entrance and exit, border crossing, airport and seaport. But large scale, invasive inspections are not economically feasible. Such rigorous inspections would result in the aforementioned disruption and instability, thereby achieving the very goal of the terrorists. Accordingly, there has been developed various methods and apparatus, as disclosed in the '154 and '307 Applications, for alternative, non-invasive “on the fly” inspection of shipping containers at the major transit points of world trade.
a. Active Versus Passive Systems and Methods
A nuclear weapon, whether designed to destroy through the explosive release of nuclear energy or through the spread of nuclear material by conventional explosive force, contains a radioactive or fissile material. As is known, radioactivity is the spontaneous emission of radiation. This radiation occurs either directly from unstable atomic nuclei or as a consequence of a nuclear reaction, and comprises alpha particles, nucleons (protons and neutrons), electrons and gamma rays. This radiation can be detected through active or passive systems and methods.
As described in the '154 and '307 applications, gamma rays emitted by radioactive or fissile material in a shipping container can be detected by various types of radiation detection devices. It is also well known in the art that neutrons emitted by radioactive or fissile material in a shipping container can be detected by various radiation detection devices. See, e.g., Perkins et al., Method and apparatus for detecting neutrons, U.S. Pat. No. 5,680,423 (“Perkins et al.”).
Active radiation detection involves (1) a source of radiation, e.g., x-rays or neutrons; (2) a radiation detection device; and, (3) in most schemes, the placement of the suspect container (or target object) between the source and the detection device in the path of the radiation emanating from the source. See, e.g., Armistead, Single Beam Photoneutron Probe And X-Ray Imaging System for Contraband Detection and Identification, U.S. Pat. No. 5,838,759. In the active system or method, the source of radiation illuminates or is focused at the container either to (1) cause a shadow of the shipping container contents to be cast on the detector (as in medical x-ray or tomography imaging); or (2) cause the target objects in the container to give off other types of radiation which are then detected by the detector.
There is an inherent fundamental problem with the known active, non-invasive detection systems and methods. The radiation source that illuminates or is focused upon the container gives off radiation which is itself harmful to people and property. As stated in the '307 Application: “Active detection systems (1) produce harmful radiation and thus are not suitable for the work environment; (2) can damage certain types of cargo carried in containers such as photographic film; (3) require a separation space between the radiation source and the detector into which space the inspected container must be placed; and, (4) are difficult to accommodate on the hoist attachment of the container crane.”
Passive detection systems, as of the type described in the '154 and '307 Applications, do not rely upon any source of radiation, but instead assess the contents of the container by reliance upon the radiation spectrum or energy detected. As described therein, an unexpected spectrum or energy could be used to subject the container to further non-invasive or invasive inspection.
b. Shielded and Unshielded Nuclear Weapons
An unshielded nuclear weapon in a shipping container can be detected by many gamma-ray and neutron detection systems and methods. As pointed out in the '154 and '307 Applications, as early as 1911, Geiger-Mueller counters were available to detect the presence of ionizing radiation. Most modern, passive radiation detection devices utilize scintillators to detect the count and energy of the gamma rays. Among other things, gas filled chambers are used to detect the presence of neutrons. For an example of detection of unshielded radiation, “portal monitors” are radiation detectors mounted on a squared-off A-frame structure. A shipping container is driven through the portal monitor. The container thereby comes into proximity to the radiation detectors and they detect the presence of the radiation by virtue of the gamma-ray and/or neutron emissions from the container.
However, as also pointed out in the '154 and '307 Applications, it is well know in the art that matter significantly attenuates gamma rays. For example, two to three inches of lead surrounding, for example, forty (40) pounds of highly enriched uranium (“HEU”) will almost entirely attenuate the gamma-ray emissions of the HEU. Any rogue nation or terrorist clever and resourceful enough to acquire a nuclear weapon in the first place will know that the best way to deliver it by stealth is to shield it. Consequently, there exists a need for improved methods and apparatus for passive non-invasive inspection of shipping containers that will detect the presence of radiation shielding material concealing impermissible quantities of radioactive or fissile material in a shipping container.